It is well known in the art that the molecular weight of polyolefin compositions, particularly those primarily composed of polymers or blends of polymers made from alpha-olefins, may be lowered by heating the polyolefins in the presence of a free radical generator such as an organic peroxide. This process of altering the polymer is commonly referred to as "degradation." Other nomenclature such as vis-breaking, chemical cracking and controlled rheology have also come into broad use in describing the degradation process. Polymer thus modified may be collected as pellets and fabricated into a variety of products including textiles, films, extruded goods and molded goods. The modified polymer is easier to process than its parent material and exhibits important property improvements such as reduced die swell and reduced brittleness.
While it is possible to degrade polyolefins merely by heating them, this method of degradation is limited because polymers will thermally decompose at temperatures as low as about 260.degree. C. It is also possible to chemically induce degradation of polymers with free radicals produced by the decomposition of free radical generators, such as organic peroxides. Treatment of a polymer with these radicals leads to radical attack on the polymer and associated chain scission. Polymer degradation through the use of free radical generators favors production of shorter, lower molecular weight chains and a net narrowing of the molecular weight distribution. These properties are all considered desirable when the polymers are to be used commercially.
For various practical reasons, the commercial producer of polymers typically does not directly measure the polymer's molecular weight, but instead depends on indirect representations of it, the most common of which are the polymer's melt flow, as measured by ASTM D 1238, Condition L; the polymer's melt index, as measured by ASTM D 1238, Condition E; and the polymer's viscosity. In general, as a polymer's molecular weight decreases, its melt flow and melt index increase and its viscosity decreases.
Certain polymers do not degrade upon treatment with free radicals, but instead exhibit an increase in molecular weight. This is because the starting polymer contains crosslinkable moities. An increase of the molecular weight in these crosslinked polymers is actually a desired property. The process disclosed herein is also useful for producing higher molecular weight in these crosslinked polymers.
While the use of free radical generators such as organic peroxides in the degradation of polymers is well established, the process is expensive, especially when large changes in the molecular weight of polymer are desired. In order to effect large changes in polymer viscosity, a commensurately large amount of free radical generator must be added. These free radical generators are generally expensive, making the cost of processing the polymer prohibitively high. Thus there is a need for a process for the degradation of polymers that is highly efficient in its consumption of free radical generators.
In addition to the high processing costs associated with degrading polyolefins using free radical generators, there are operational problems associated with using free radical generators for producing degraded polyolefins. For example, it is generally known that products made from polymers that were subjected to a large change of viscosity may suffer from non-uniformities of composition that make the product unsuitable for commercial use in article fabrication. Additionally, excessive dosing of free radical generators during the preparation of the polymer can cause surging, foaming, difficulty in pelletization and other processing problems. These processing problems are largely overcome when the polymer is prepared according to the teachings of the present invention.